Boonton 4541, Instruction Manual

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Boonton 4540 Series RF Power Meter
Application Notes
6-22
Step 10:
Now that each of the individual uncertainty terms has been determined, we can combine them to calculate the worst-
case and RSS uncertainty values:
U (±%) K (U×K) 2 ( % 2 )
1. instrument uncertainty 0.10 0.500 0.0025
2. calibrator level uncertainty 2.45 0.577 1.9984
3. calibrator mismatch uncertainty 0.34 0.707 0.0578
4. source mismatch uncertainty 6.68 0.707 22.305
5. sensor shaping error uncertainty 1.00 0.577 0.3333
6. sensor temperature drift uncertainty 0.00 0.577 0.0000
7. sensor noise uncertainty 0.95 0.500 0.2256
8. sensor zero drift uncertainty 1.58 0.577 0.8311
9. sensor calibration factor uncertainty 4.09 0.500 4.1820
___________________________
Total worst case uncertainty: ±17.19%
Total sum of squares: 29.936 %
2
Combined Standard uncertainty U
C
(RSS) : ±5.47 %
Expanded Uncertainty U (coverage factor k = 2) : ±10.94 %
From this example, it can be seen that the two largest contributions to total uncertainty are the source mismatch, and the
sensor calfactor. Also note that the expanded uncertainty is approximately one-half the value of the worst-case uncertainty.
This is not surprising, since the majority of the uncertainty comes from just two sources. If the measurement frequency was
lower, these two terms would be reduced, and the expanded uncertainty would probably be less than half the worst-case.
Conversely, if one term dominated (for example if a very low level measurement was being performed, and the noise
uncertainty was 30%), the expanded uncertainty value would be expected to approach the worst-case value. The expanded
uncertainty is 0.45dB.
Typical Example #2: Model 57518 Peak Power Sensor
4540 Series measurement conditions:
Source Frequency: 900 MHz
Source Power: 13 dBm (20mW)
Source SWR : 1.12 (reflection coefficient = 0.057) at 900 MHz
AutoCal Source: External 1GHz Calibrator
AutoCal Temperature: 38C
Current Temperature: 49C
In this example, we will assume that an AutoCal was performed on the sensor earlier in the day, so time and temperature drift
may play a role in the uncertainty.
Step 1:
The Instrument Uncertainty figure for the 4540 Series is ±0.20%. Since it has been a while since AutoCal, we’ll use
the published figure.
U
Instrument
= ±0.20%